In U.S. Pat. No. 3,868,920 there is described a cargo vessel having a submergible cargo carrying hull and a bridge from which the vessel is operated supported above the surface by a mast projecting from the hull. The vessel therein described is designed to operate in ice covered waters by submerging the hull below the surface of the ice with the bridge supported above the surface of the ice by the mast. Rotary ice cutters extending along the front edge of the mast operate to cut a path in front of the mast. The vessel therein described serves to transport petroleum and other mineral values from arctic shores to the consuming areas of the United States and other parts of the world in a safe, economical and year-round transportation system.